1. Field of the Invention
The invention relates to high fidelity audio amplifiers and more particularly to an active bias circuit for achieving essentially Class A operation in push-pull transistor amplifiers.
2. Prior Art
Low distortion, wide bandwidth, high power, and unconditional stability are the most important performance features desired in high fidelity power amplifiers. Present solid-state designs easily attain 100 watts over bandwidths greater than 20 to 20,000 hertz with less than 0.1% total rated harmonic or intermodulation distortion. However, many amplifiers which meet such specifications sound poorer than amplifiers with nearly identical specifications or even older devices with 5 times more rated distortion.
Part of the problem arises because of the rating method. Distortion and power tests are invariably made into resistive loads, usually eight ohms, while actual speaker systems have resistive, reactive and dynamic impedance components. Amplifiers generally have poorer distortion features into reactive and dynamic loads, and especially poorer figures into capacitive loads.
Distortion is usually computed by taking the square root of the sum of the squares of the amplitude of each harmonic. Thus, the total figure does not reveal the distribution of distortion components. It is well known that it is desirable to concentrate distortion second and third harmonics where it can be masked by the harmonic content of the program. Harmonics greater than third order harmonics are less subtle and create a harsh quality to the audio output.
Lower order harmonics are caused by gradual or "smooth" non-linearities in the amplifying system, whereas higher order harmonics are caused by more abrupt non-linearities. Thus, to avoid higher order harmonics, abrupt non-linearities must be eliminated. Most solid-state audio equipment, while having very low total harmonic distortion, is often characterized by many higher order distortion components, resulting in what is known as "transistor sound". Older tube amplifiers, while having higher distortion, have the distortion in lower order harmonics and such tube equipment characterized by "tube sound", is preferable to many audiophiles, for all of the drawbacks in tube type equipment.
The abrupt non-linearities in the characteristic curves of transistor amplifiers arise primarily because of what is known as "cross-over distortion". Cross-over distortion arises in Class B, push-pull amplifiers. This is an amplifier type using two complementary transistors, often arranged in an emitter follower configuration. For example, see "Electronic Devices and Circuits" by Jacob Millman and Christos Halkins, McGraw-Hill, 1967, p. 563, FIG. 18-14, showing a push-pull emitter follower audio output circuit with transistors having complementary symmetry. FIG. 18-15 illustrates cross-over distortion. Professor Millman says that cross-over distortion "may be decreased by biasing the transistors at a higher standby current. A compromise must be made between distortion and efficiency."
The disadvantage of Class B operation can be remedied by utilizing Class A operation in which the operating region of the transistor contains the least abrupt non-linearities. However, the essential disadvantage of Class A circuits is their general inefficiency, since the bias current is usually as great or greater than the peak output current. Thus, most of the power delivered to the circuit is dissipated in the output stage of the amplifier as heat.
Another remedy is to use Class AB operation. In that mode, forward bias is provided to both output transistors in a circuit similar to the complementary symmetry emitter follower described above, when the amplifier is in a quiescent mode. Class AB operation does much to eliminate abrupt crossover non-linearity by maintaining a forward bias across both of the push-pull transistors at output current generally less than the bias current. However, the amount of bias current which can be supplied is limited by thermal dissipation capabilities of the output transistors.
In conventional amplifiers using emitter-follower output stages in complementary symmetry, bias current is developed by use of a constant voltage generator whose voltage output is equal to the sum of the forward bias voltage drops of the emitter followers in series plus the IR drops of the series resistances in the bias path, see FIGS. 1 and 2, wherein FIG. 1 shows a constant voltage generator of the prior art and FIG. 2 shows a use of this bias circuit with a complementary symmetry output stage of the prior art.
The apparatus of FIG. 1 generates a relative constant bias voltage, V.sub.B, across bias feed nodes A and B: ##EQU1## This equation is derived by noting the voltage across R2 is equal to V.sub.BE. The current flowing through R1 is assumed to be the same current flowing through R2, neglecting the small base current of the transistor so that the current through R2 is equal to V.sub.BE /R2.
The bias transistor, Q2, of a V.sub.BE Multiplier is mounted in thermal contact with the output transistors for temperature compensation.
Sometimes the constant voltage generators of the prior art used some form of forward biased diode junction and were designed for minimal bias voltage fluctuation over various dynamic conditions of the output stage of the amplifier. The use of diode junctions allowed good thermal tracking of bias current by placing the constant voltage bias generator in thermal contact with the output transistors.
Although Class AB operation is beneficial and improves the problem of abrupt non-linearities in transistor amplifiers, cross-over distortion is still found and is a higher percentage of the waveform content as power decreases. The cross-over non-linearities still exist when output transistors of the Class AB stage turn on and off. Feedback is used to add still greater linearity to the output.
It is an object of the present invention to build an efficient amplifier using Class AB emitter follower complementary output stages, but eliminating cross-over distortion by operating the circuit in a Class A mode over the entire current and voltage output swing.